In recent years, intercom systems that enable communication between a master device and a slave device have been utilized to achieve a crime prevention effect. A conventional intercom system has a slave device, a master device that can respond to a call from the slave device, and an auxiliary master device that can respond to a call from the master device, and both the master device and the auxiliary master device are equipped with a microphone. If a sound picked up by either of the microphones is abnormal, the master device sends a notification corresponding to the abnormal state to an external terminal. Such technology is disclosed in Japanese Patent Publication No. 3879641, for example.